This invention relates to antennas, and more particularly to an electronically scanned antenna incorporating a plurality of radiating elements which form the antenna, and which each incorporate a plurality of layers of switchable devices for providing a requisite degree of phase shift to an electromagnetic signal received and reflected from each radiating element in order to form a beam and to point it in a desired direction.
Radar and communication systems require antennas to transmit and receive electromagnetic (EM) signals, generally in the microwave or millimeterwave spectrum. One class of antennas is the electronically scanned antenna (ESA). In an ESA, the signal is transmitted and received through individual radiating elements distributed uniformly across the face of the antenna. Phase shifters in series with each radiating element create a well-formed, narrow, pencil beam and tilt its phase front in the desired direction (i.e., xe2x80x9cscanxe2x80x9d the beam). A computer electronically controls the phase shifters. ESAs offer fast scan speeds and solid state reliability.
While ESAs have proven effective in many applications, the main deterrent to their widespread application is their high cost. Another drawback is that ESAs have higher insertion losses associated with their phase shifters than mechanically scanned antennas. These losses increase the output power required of the transmitter of the ESA, which in turn increases its cost, power supply requirements and thermal management due to the increased power dissipation.
One approach to overcome the loss issue mentioned above is the use of an active ESA (AESA). The AESA is constructed by pairing amplifiers with phase shifters in the antenna. An AESA incorporates a power amplifier to provide the requisite transmitted power, a low noise amplifier to provide the requisite receiver sensitivity and a circulator connecting the transmit and receive channels to the radiating element. This approach is viable for small arrays, i.e., arrays of a few hundred elements. However, for a given antenna size, the number of radiating elements increases as the square of the frequency. Thus, for a high gain, millimeter wave antenna, the array often contains thousands of elements. In this instance, cost, packaging, control, power distribution and thermal management issues become significantly important concerns.
Space fed configurations using a passive ESA (PESA) promise to be less expensive than active ESAs for millimeter wave applications. A passive ESA does not use distributed amplifiers, but instead relies on a single high power transmitter and a low loss antenna. The reason for the lower cost is the simpler architecture of such an antenna that has fewer, less expensive parts. A PESA can be implemented in a number of quasi-optic configurations such as a focal point or off-set J-feed reflection antenna, as a transmission lens antenna, as a reflection Cassegrain antenna, or as a polarization twist reflection Cassegrain antenna. However, since PESAs do not have amplifiers to overcome the circuit losses, these losses, and particularly the phase shifter loss, become a key issue.
An approach to reduce phase shifter insertion loss is to implement the phase shifter with a micro-electromechanical system (MEMS) switch. The MEMS switch can be employed as the control device in various types of phase shifter designs. Since it is an electromechanical switch, it offers low insertion loss. A microwave monolithic integrated circuit (MMIC) of MEMS-based phase shifters and radiators can be fabricated as a subarray. This scale of integration promises lower costs. However, MEMS based MMIC phase shifters remain expensive and their integration into a full array will be even more costly for a millimeter wave antenna. They are also relatively fragile compared to solid state devices and require high control voltages, for example, about 70 Volts. For some configurations, packaging the phase shifter and radiator(s) in the requisite cell area, the maximum area that a radiating element can occupy for proper operation over a given maximum frequency and scan angle, is also difficult.
In view of the foregoing, it is a principal object of the present invention to provide a simpler, less lossy, more cost-effective solution for an ESA requiring large pluralities of radiating elements.
More particularly, it is a principal object of the present invention to provide an ESA in which the entire antenna aperture can be fabricated and assembled at the wafer level.
The above and other objects are provided by a layered electronically scanned antenna (LESA) and method in accordance with the preferred embodiments of the present invention. The LESA is structured as two or more layers of thin wafers consisting of uniformly distributed cells of solid-state components and separated by dielectric spacers. The size and pattern of the distribution is determined by known antenna array theory. The components of each layer are coincident with each other so that the cells on each layer form an array element of the antenna. Each component can operate either as a reflective cell or as a transmissive cell. Each reflective/transmissive component consists of a plurality of switch devices that control whether the cell assumes a reflective or transmissive state. The dielectric spacers between the wafer layers each have a predetermined electrical thickness that, together with the wafer layer, provides a desired degree of spatial phase shift to an electromagnetic wave passing therethrough. In one preferred embodiment each spacer has an electrical thickness of 45xc2x0. Thus, an electromagnetic wave passing through the wafer layer and spacer and being reflected back through the spacer and wafer layer by a reflective component undergoes a 90xc2x0 phase shift referenced to the face of the array. A control circuit controls the switch devices within the components such that the components of each cell at each layer are made to be either reflective or transmissive to thus achieve the desired degrees of phase shift of the signal radiating from each array element. Again by known antenna array theory, the distribution of phase settings across the array can be determined to form and point the beam of the radiated signal to a given direction. Additionally, if the main reflector that contains the array is flat, then the phase shift of each array element can be set to provide an electrical parabolic shape in order to properly focus the beam.
In one preferred embodiment, each array element includes three wafer layers separated by three dielectric spacers each having an electrical thickness of 45xc2x0. A fourth layer is included which is strictly a reflective layer, typically a metal sheet (ground plane). If a cell on the first layer is in a reflective state, then the signal incident thereon is reflected with a given phase that establishes the reference or zero phase state at the face of the array. If the cell on the first layer is in a transmissive state, then the electromagnetic signal passes therethrough, through the first spacer and impinges on the cell on the second layer directly beneath the cell on the first layer. If the cell on the second layer is in the reflective state, then the electromagnetic signal is reflected therefrom back through the first spacer and the first layer to provide a phase shift of 90xc2x0 with respect to the face of the array. If the cells on the first and second layers are both in the transmissive state, and if the cell in the third layer is in the reflective state, then a phase shift of 180xc2x0 will be imparted to the signal reflected therefrom as the signal makes two passes through the two layers and spacers. If the cells on all three layers are in the transmissive state, then the signal is reflected by the ground plane back through the three spacers and layers to provide a phase shift of 270xc2x0.
The LESA of the present invention can be implemented in a wide variety of configurations including focal point or off-set J-feed, reflection Cassegrain, polarization twist reflection Cassegrain and other known antenna configurations. The reflective/transmissive components can be formed by a group of resonant dipoles, resonant cross dipoles (cruciforms) or other components that fit within a cell and provide a reflective surface at the operating frequency when disconnected from each other and a transmissive surface when connected to each other.
The present invention thus constructs an affordable millimeter wave ESA employing wafer level fabrication and assembly. It enables an ESA to be constructed without the need for MEMS switches to achieve low insertion loss at millimeter wave frequencies. However, the invention does not preclude their use or the use of any other type of switch. It further provides for a reliable, solid-state ESA that can be driven by low voltage CMOS control circuitry.